A fuel cell device generates electricity directly from a fuel source, such as hydrogen gas, and an oxidant, such as oxygen or air. Since the process does not "burn" the fuel to produce heat, the thermodynamic limits on efficiency are much higher than normal power generation processes. In essence, the fuel cell consists of two catalytic electrodes separated by an ion-conducting membrane. The fuel gas (e.g. hydrogen) is ionized on one electrode, and the hydrogen ions diffuse across the membrane to recombine with the oxygen ions on the surface of the other electrode. If current is not allowed to run from one electrode to the other, a potential gradient is built up to stop the diffusion of the hydrogen ions. Allowing some current to flow from one electrode to the other through an external load produces power.
The membrane separating the electrodes must allow the diffusion of ions from one electrode to the other, but must keep the fuel and oxidant gases apart. It must also prevent the flow of electrons. Diffusion or leakage of the fuel or oxidant gases across the membrane leads to explosions and other undesirable consequences. If electrons can travel through the membrane, the device is fully or partially shorted out, and the useful power produced is eliminated or reduced.
It is therefore an object of this invention to produce a membrane which allows the diffusion of ions, but prevents both the flow of electrons and the diffusion of molecular gases. The membrane must also be mechanically stable.
In constructing a fuel cell, it is particularly advantageous that the catalytic electrodes be in intimate contact with the membrane material. This reduces the "contact resistance" that arises when the ions move from the catalytic electrode to the membrane and vice versa. Intimate contact can be facilitated by incorporating the membrane material into the catalytic electrodes. [See Wilson and Gottsfeld J. Appl. Electrochem. 22, 1-7 (1992)] It is therefore an object of the invention to produce a membrane wherein such intimate contact is easily and inexpensively made.
For reasons of chemical stability, fuel cells presently available typically use a fully fluorinated polymer such as Dupont Nafion.RTM. as the ion-conducting membrane. This polymer is very expensive to produce, which raises the cost of fuel cells to a level that renders them commercially unattractive. It is therefore a further object of this invention to produce an inexpensive ion-conducting membrane.
Ion-conducting polymers are known. (See Vincent, C. A., Polymer Electrolyte Reviews I, 1987). The known polymers are, for the most part, similar to sulfonated polystyrene because of the known ability of sulfonated polystyrene to conduct ions. Unfortunately, uncrosslinked, highly sulfonated polystyrenes are unstable in the aqueous environment of a fuel cell, and do not hold their dimensional shape.
U.S. Pat. No. 4,849,311 discloses that a porous polymer matrix may be impregnated with an ion-conducting polymer to produce a fuel cell membrane. However, the ion-conducting polymer must be dissolved in a solvent which "wets" the porous polymer. When the solvent evaporates, there is sufficient porosity remaining in the porous polymer/ion-conducting polymer composite material that molecular oxygen can leak through to the fuel gas and result in an explosion.
U.S. Pat. No. 3,577,357 (Winkler) discloses a water purification membrane composed of block copolymers of sulfonated polyvinyl arene block and alpha-olefin elastomeric blocks. In one example a styrene-iosprene-styrene triblock copolymer was selectively hydrogenated, then sulfonated using a premixed SO.sub.3 /triethylphosphate reagent at 60.degree. C. for 1.5 hrs. A sulfonated styrene-(ethylene-propylene) copolymer was the result. The method provided solid agglomerates of the polymer which were rolled on a mill to remove water, swelled in cyclohexane, slurried in an isopropyl alcohol/water mixture, and coagulated in hot water. No membrane was produced, and we have found that polymers produced according to the method of Winkler cannot be cast into films.
Gray et al. [Macromolecules 21, 392-397 (1988)] discloses a styrene-butadiene-styrene block copolymer where the ion-conducting entity is a pendant short-chain of poly(ethylene oxide) monomethyl ether (mPEG) complexed with LiCF.sub.3 SO.sub.3 salt and connected through a succinate linkage to a flexible connecting entity which is the butadiene block of the triblock copolymer. The ion-conducting entity in the butadiene block is in the continuous phase of the polymer, and the areas populated by the ion-conducting entities do not preferentially touch each other to form continuous ion-conducting domains. This morphology does not facilitate the ion-conducting properties that are necessary for fuel cell operation. The styrene block functions only as a mechanical support structure for the polymer. Moreover, the molecular design chosen by Gray et al. is incompatible with the working environment of a fuel cell. Because the succinate linkage which joins the mPEG to the butadiene backbone and the ether linkages which join the ethylene oxide units are subject to cleavage by acid hydrolysis, these linkages are unstable in the low pH environment of a fuel cell even for short periods of time.
In the art of battery separators, as exemplified by U.S. Pat. No. 5,091,275, a number of porous polymers and filled polymer materials are known. The pores of these polymers and composite materials are filled with, typically, a liquid electrolyte to conduct ions from one electrode to another in a battery. However, these battery separator materials allow the passage of gases, so that fuel cells made with them have an unfortunate tendency to explode as the oxygen leaks into the hydrogen side of a fuel cell.
There is therefore a need for an inexpensive, mechanically and chemically stable, ion-conducting membrane.